Recently, a display device is indispensable as a man-machine interface, and among this kind of display devices, a liquid crystal display device is superior since it is thin, lightweight, low in power consumption, and color-pictured. Among them, a passive matrix liquid crystal display device is used widely because the price and so forth are within the range of acceptance.
Conventionally, a passive matrix liquid crystal display device is driven by ALT PLESHKO Technique which conducts line-at-a-time scanning of scanning lines. This method is described in detail in "Scanning Limitations of Liquid-Crystal Display, ALT P. M. and Pleshko P., IEEE Trans. Ed. Vol. ED 21, pp 146-155 (1974)". However, when this method is applied to a high-speed response liquid crystal panel, on-state brightness drops due to a frame response, so that contrast deteriorates. Therefore, for preventing this kind of contrast deterioration, a driving method proposed lately does not conduct line-at-a-time scanning, but selects a total number or a plurality of scanning lines simultaneously.
In the following, the driving method of selecting a total number or a plurality of scanning lines simultaneously will be explained. When liquid-crytal drive is perceived mathematically, it can be shown as Formula 1 below. EQU Y=M.multidot.X (Formula 1)
In Formula 1 mentioned above, X represents a matrix of picture image data, and on-state is indicated as "-1", while off-state is indicated as "1". Furthermore, M represents a matrix of scan data, and a selected condition is indicated as either "1" or "-1", while a non-selected condition is indicated as "0". Then, Y which is operated by this Formula 1 becomes a matrix of signal data. However, for the signal data to be in proportion to the picture image data, the matrix of scan data M needs to be an orthogonal matrix.
Here, when each element in the matrix of scan data M is indicated as m, each element in the matrix of picture image data X is indicated as x, and each element in the matrix of signal data Y is indicated as y, signal data y.sub.ij of an (i, j) pixel within one frame is shown as Formula 2 below. ##EQU1## In Formula 2 mentioned above, N represents a total number of rows in the matrix of picture image data X, and t represents time.
In addition, when voltage for one level in the matrix of signal data Y is indicated as V.sub.b and k as a constant, voltage on scan side V.sub.r to the (i, j) pixel within one frame is shown as Formula 3 below. EQU Vr=km.sub.ti .multidot.Vb (Formula 3)
Moreover, voltage on signal side V.sub.c to the (i, j) pixel within one frame is shown as Formula 4 below. EQU Vc=y.sub.ti .multidot.Vb (Formula 4)
By using the above-mentioned Formula 1, Formula 2, Formula 3, and Formula 4, applied effective voltage V.sub.ij to the (i, j) pixel can be obtained as shown in Formula 5 below. ##EQU2## In Formula 5 mentioned above, N represents a total number of rows in the matrix of picture image X; S represents a number of elements besides "0" in an optional row of the matrix of scan data M (hereinafter referred to as a "number of simultaneously selected lines"); and t represents time. According to Formula 5, when total elements in the matrix of picture image data X are either "1" or "-1", as shown in Formula 6 below, the third term in Formula 5 becomes a total number of rows N (constant) in the matrix of picture image data X, and dependency of an element x.sub.ij in the matrix of (i, j) picture image data upon the applied effective voltage V.sub.ij will be the second term in Formula 6 only, so that effective voltage in proportion to the element x.sub.ij in the matrix of (i, j) picture image date will be applied. ##EQU3## In the following, a method of driving a liquid crystal display device will be explained by means of the above-mentioned conventional driving method of selecting a total number or a plurality of scanning lines simultaneously.
FIG. 20 is a drawing which shows a display operation method in the conventional driving method of selecting a total number or a plurality of scanning lines simultaneously. In this figure, reference numeral 10 represents a matrix of scan data; 20 represents a matrix of picture image data; 30 represents a matrix of signal data; 50 represents a maximum value of signal data; and 60 represents an operation order. As an example used here is the matrix of scan data 10 in 248-order which is a circulant Hadamard matrix in eight-order shown in Formula 7 below (a number of simultaneously selected lines S=8) having inverted signs for each row and each column and being extented by Kronecker product with a unit matrix in 31-order. ##EQU4##
Furthermore, the matrix of picture image data comprises 240 rows and 2 columns (N=240), and each element in the first column is "-1" and "1" being repeated from the first row to the N row, and "-1" is inserted as dummy data into the (N+1) row and "1" from the (N+2) row to the (N+8) row. In the second column, "1" is inserted totally from the first row to the N row, and dummy data of "1" is inserted from the (N+1) row to the (N+8) row. Accordingly, the matrix of picture image data 20 is comprised of 248 rows and 2 columns as a whole. In this case, the matrix of signal data 30 is constructed by an operation in an order shown in the operation order 60. Also, the signal data reaches the maximum when each element in the row of the matrix of scan data 10 and each element in the column of the matrix of picture image data 20 conform completely, and this value is "8".
Next, a configuration of a conventional apparatus for driving a passive matrix liquid crystal display device will be explained which can be applied to the conventional driving method of selecting a total number or a plurality of scanning lines simultaneously by means of the above-mentioned operation method. Also, its operation will be explained.
FIG. 21 is a block diagram showing a conventional apparatus for driving a passive matrix liquid crystal display device. As shown in FIG. 21, the conventional apparatus for driving a passive matrix liquid crystal display device is comprised of a field memory of picture image data 70 for storing picture image data being input from outside; a readout circuit of picture image data 71 for reading out each element in a specific column of a matrix of picture image data; a memory of scan data 80 for storing scan data in advance; a readout circuit of scan data 81 for reading out specific scan data from the memory of scan data 80; an operation circuit of each element 90 for operating a matrix of signal data Y based on picture image data in a specific column being read out from the field memory of picture image data 70 and also based on scan data being read out from the memory of scan data 80; a field memory of signal data 100 for storing data after being operated; a readout circuit of signal data 101 for reading out operated signal data from the field memory of signal data 100; a driver on scan side 110; a D/A converter 120 for converting the read-out data signals from digital signals to analog signals; a driver on signal side 130; a passive matrix liquid crystal display device 140; and a frame reducing controller 150 for controlling gradation (hereinafter referred to as a "FRC").
After picture image data being input from outside is input into the FRC 150, gradation control is conducted by the FRC 150. The picture image data performed with the gradation control is once stored in the field memory of picture image data 70. Then, each element in the first column of the matrix of picture image data 20 is read out by the readout circuit of picture image data 71, which is then operated by the operation circuit 90 by using scan data being stored in the memory of scan data 80 and by solving Formula 1 mentioned above. At this time, each element in the matrix of scan data 10 is read out in order from the first row to the 248th row by the readout circuit of scan data 81. This operation is conducted in the similar manner in the second column of the matrix of picture image data 20.
After being operated, the data is output according to the operation order 60 shown in FIG. 20 and then stored in the field memory of signal data 100. Next, the data is read out by the readout circuit of signal data 101 in the order of transfer to the driver on signal side 130, and after being converted from digital signals to analog signals by the D/A converter 120, the data is transferred to the driver on signal side 130. The driver on signal side 130 applies voltage in accordance to the analog signal data being input to an electrode on signal side in the passive matrix liquid crystal display device 140. On the other hand, on scan side, the operated data is stored in the memory of scan data 80, and each element in the matrix of scan data is read out in order from the first row to the 248th row by the readout circuit of scan data 81, which is then transferred to the driver on scan side 110. The driver on scan side 110 applies voltage in accordance to the scan data being input to an electrode on scan side in the passive matrix liquid crystal display device 140.
According to the method mentioned above, by increasing a number of simultaneously selected lines (a number of scanning lines being selected) and dispersing effective voltage which is imposed on each pixel within one frame, a frame response in a high-speed liquid crystal is suppressed, so contrast can be improved. This method is described more in detail in "Hardware Architectures for Video Rate, Active Addressed STN Displays, B. Clifton etc. JAPAN DISPLAY'92 pp. 504-506".
However, in order to make the applied effective voltage V.sub.ij to the (i, j) pixel to be in proportion to the element x.sub.ij in the matrix of (i, j) picture image data as mentioned above, each element x in the matrix of picture image data must be conditioned to be either totally "1" or "-1". The reason is that if each element x in the matrix of picture image data were not totally "1" or "-1", the third term in Formula 4 mentioned above would not be a constant. Therefore, when each element x in the matrix of picture image data is in the range of "-1" to "1" and performs a gradation display having a value besides "1" and "-1", the third term in Formula 4 is not a constant, but becomes a term dependent upon each element x in the matrix of picture image data, similar to the second term. Thus, the applied effective voltage V.sub.ij to the (i, j) pixel is no longer in proportion to the element x.sub.ij in the matrix of (i, j) picture image data. As mentioned above, in the conventional driving method of selecting a total number or a plurality of scanning lines simultaneously, a gradation control by a pulse height of the applied voltage can not be conducted, so that for a gradation display, it was necessary to conduct a gradation control by a frame rate control method (hereinafter referred to as a "FRC system"). As a result, the quality of display was ruined due to flickers occuring in the image plane. Another problem as follows arises when a circulant Hadamard matrix is used as a matrix of scan data.
FIG. 19 shows an example of a waveform of applied voltage in liquid crystal and a waveform of an optic response in liquid crystal, provided that a circulant Hadamard matrix consisting of 420 rows and 420 columns having signs reversed for every row and for every column is used, and that data at on-state is included in the column direction of a matrix of picture image data, and that an abscissa at the display of off-state indicates time. At this time, however, a response speed of the liquid crystal was 150 msec for rising and decaying in average. In this figure, reference numeral 222 represents an observed waveform of an optic response in liquid crystal; 223 represents an ideal waveform under the same conditions; 210, 211 represent ground; and 224 represents a waveform of applied voltage to liquid crystal. In this case, the observed waveform of an optic response in liquid crystal 222 shows negative electrode property against the ground 210.
In addition, FIG. 18 shows an example of a waveform of applied voltage in liquid crystal and a waveform of an optic response in liquid crystal, provided that a circulant Hadamard matrix consisting of 420 rows and 420 columns having signs reversed for every row and for every column is used, and that only data at off-state is present in the column direction of a matrix of picture image data, and that an abscissa at the display of off-state indicates time. A response speed of the liquid crystal was 150 msec for rising and decaying in average. In this figure, reference numeral 218 represents an observed waveform of an optic response in liquid crystal; 219 represents an ideal waveform under the same conditions; 220 represents a pulsative response part; and 221 represents a waveform of applied voltage to liquid crystal. In FIG. 18, the same reference numerals are given to the parts which are identical to those in FIG. 19, and the explanation is omitted. Also in this case, the observed waveform of an optic response in liquid crystal 218 shows negative electrode property against the ground 210.
As shown in FIG. 18, it is clear that a periodic change of low frequency can be observed in the waveform of applied voltage to liquid crystal 221, and that the observed waveform of an optic response in liquid crystal 218 has the pulsative response part 221 when a display at off-state is performed, and that brightness has enhanced against the ideal waveform 219. In FIG. 19, on the other hand, although a pulsative response part can be observed in the observed waveform of an optic response in liquid crystal 222, the degree is small, so the waveform has become closer to the ideal off-brightness.
Accordingly, when the circulant Hadamard matrix having signs reversed at an equal rate as shown in Formula 7 above is used, cross-talk occurs due to a difference in brightness at offstage caused by the content of the picture image data. Furthermore, there was a problem that since the off-brightness does not drop, the contrast does not improve as well.
Incidentally, when a matrix of signal data is operated by using a matrix of scan data which is an orthogonal matrix consisting of three values of "1", "0", and "-1", among these orthogonal matrixes with three values, a displayed picture image having higher contrast can be obtained by using a matrix T' shown as Formula 9 below, rather than using a matrix T shown as Formula 8 below. ##EQU5##
The matrix T shown as Formula 8 above can be obtained by extending an orthogonal matrix S consisting of two values of "1" and "-1" as shown in Formula 10 below (hereinafter referred to as a "sub matrix") by Kronecker product shown in Formula 12 below with the use of a unit matrix I shown as Formula 11. ##EQU6##
Also, the matrix T' shown as Formula 9 above can be obtained by using i and i' obtained by Formula 13 below and by changing the i row in the matrix T shown as Formula 8 above to the i' row. EQU i=r.times.n+s+1 EQU i'=s.times.m+r+1 (Formula 13)
(i, i': a natural number less than or equal to N; r: an integral number greater than or equal to 0, and less than m; s: an integral number greater than or equal to 0, and less than m) In Formula 13 mentioned above, n represents a degree of the sub matrix S, and m represents a degree of the unit matrix I.
The reason why the above-mentioned difference of contrast occurs is as follows. Namely, since the longitudinal direction of a matrix of scan data 301 shown in FIG. 11, which is a drawing showing the relationship with a liquid crystal panel, corresponds to the time direction, an interval between one selective period of 1, -1 to the next selective period of 1, -1 is longer in the matrix T shown as Formula 8 above than the matrix T' shown as Formula 9 above, so that the same phenomenon as a frame response occurs.
As mentioned above, a matrix of scan data used conventionally was a matrix which can be produced by an easy operation of extending and expanding an optional sub matrix S having two value elements of "1" and "-1" to a degree which is suitable for a matrix size of picture image data with the use of an optional unit matrix I.
In this case, however, irregularity occurs in correspondence with the sub matrix S to the applied voltage of a scanning line as a unit to a liquid crystal display. As a result, an optic response in liquid crystal becomes irregular, which leads to course-marked contrast patterns in the displayed picture image, so the quality of display was ruined.